One. Power Conversion Module
The power supply voltage refers to the input voltage that is converted through a voltage conversion circuit to achieve boosting or bucking for other modules. In smart car development, the input battery voltage is 7.2V, and different modules require various voltages: the minimum system board, OLED display, and Eagle Eye camera need 3.3V; the op-amp requires ±5V; the servo motor needs 6V; the drive circuit operates at 12V; and the CCD and encoder require 5V. Below are some detailed explanations of the voltage conversion circuits.
Voltage regulator circuits are generally categorized into several types:
LM2940 for 5V:
The LM2940 is a linear voltage regulator (only for buck). It has a simple structure, high stability, but also higher power consumption, larger size, and lower filtering efficiency. It requires large input and output capacitors for proper operation.
MC34063 for 12V:
The MC34063 is a switching DC regulator chip capable of both boosting and bucking. This type of circuit offers low power consumption, high efficiency, compact size, and a wide voltage range. However, it is more complex in design and has a higher failure rate. The principle involves turning on the internal switch transistor at a certain frequency (determined by the timing capacitor), charging and discharging an inductor and capacitor to provide energy to the load. The output voltage can be calculated as Uo = 1.25 × (1 + R4/R3).
MIC29302 for 6V:
The MIC29302 is a linear adjustable regulator, similar to the MC34063, and is suitable for high-current loads. Its output voltage formula is Uo = 1.242 × (1 + R1/R2). In this case, it provides 6V to power the servo motor.
LM2663 for -5V:
The LM2663 is a simple negative voltage converter that uses only two external capacitors.
Note: Reverse connections due to manual errors should be avoided. An anti-reverse protection design can be added to the circuit.
Diode-based unidirectional conduction:
MOSFET-based anti-reverse protection circuit:
An NMOS transistor is connected to the negative terminal of the power supply, with the gate turned on at a high level. A PMOS transistor is connected to the positive terminal, with the gate turned on at a low level. (NMOS has lower on-resistance, so it is preferred.)
Various anti-reverse interfaces.
Two. Motor Drive Circuit
Although the MCU can output a DC signal, its driving capability is limited. Therefore, the MCU usually drives a high-power transistor like an MOSFET (LR7843) to generate a large current for the motor. By controlling the duty cycle of the driving signal, the average voltage applied to the motor can be adjusted, thus achieving speed control.
Motor drive typically uses N-channel MOSFETs to build an H-bridge circuit. The H-bridge is a common DC motor control circuit, named for its shape resembling the letter "H." The four switches form the vertical legs of the H, while the motor acts as the horizontal bar. To make the motor run, a pair of diagonal switches must be turned on, and the direction of current determines whether the motor rotates forward or backward. The circuit is shown below.
H-bridge drive principle:
In practical drive circuits, switches are often controlled via hardware. Motor driver boards mainly use two types of driver chips: full-bridge drivers like HIP4082 and half-bridge drivers like IR2104. The half-bridge consists of two MOSFETs, while the full-bridge uses four. The IR2104 is a popular half-bridge driver that can drive both high-side and low-side N-channel MOSFETs, providing a large gate drive current and including features such as hardware dead time and anti-same-arm conduction. Using two IR2104 chips can create a complete H-bridge motor driver circuit. Compared to HIP4082, the IR2104 is more cost-effective and function-rich, though it has lower output power.
Additionally, because the driver circuit may produce a large back-EMF current, it's advisable to isolate the chip to prevent interference with the microcontroller. Common isolation chips include 74LVC245, 74HC244, and PS2801. These chips are often used as control buses to enhance driving capability. When certain conditions are met, the output matches the input, allowing one-way signal transmission from the microcontroller to the driver chip and vice versa.
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